Quick D: Static Helicopter Blades

Captain Disillusion
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    Captain Disillusion gets pedantic about the description of a common camera effect.

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    Video being discussed: https://www.youtube.com/watch?v=yr3ngmRuGUc
    Guest - Matt Parker: http://www.youtube.com/standupmaths


    some videos get sent to me over and over even though they come pre debunked like this one as a helicopter takes off its rotor appears to barely move how well it's explained right in the title camera shutter speed matches helicopters rotor very viral also for any licensing inquiries please contact the guy but we're not here to profit off reposting the video we're here to profit off a transformative work of criticism designed to educate without detracting from the potential market for the original copyrighted work in my professional opinion the explanation in the title is wrong or at least incomplete shutter speed is only responsible for making the blades look sharp by exposing the image for a tiny fraction of a frames time motion blur is reduced but even without motion blur spinning things can look like they're spinning the reason this rotors blades look like they're not spinning is because the times skipped between video frames recorded by this camera happen to match a 72 degree multiple of the five bladed rotors turn and since helicopter rotors and camera frame rates move at a constant precisely controlled speed they stay in sync and the effect continues for a while so let's edit the videos title to be more accurate and we're done now what but can you recreate a similar video with a different helicopter linking smiley face I would love to Meg unfortunately my helicopter is currently in the shop so I'll have to demonstrate using a simpler method that you can all try at home it involves a strobe light and a ceiling fan luckily I just installed one the stroke simulates a limited framerate while flashing it at the spinning fan we can adjust the rate until it matches those 72 degree increments of rotation and see the effect in real life but showing you the process in this video presents an additional challenge because of the dreaded artifact known as rolling shutter back when cameras were old and lame they captured space-time as it really is happening all at once that's so last century today's state-of-the-art cameras capture images progressively skewing time across the vertical dimension of a frame like jello the effect of these rolling scan is always there regardless of shutter speed distorting all moving objects it's nearly invisible in most situations especially if the object is small but very rapid motion across the frame unblurred by a fast shutter and make it very apparent it bends the blades of our fan and captures the strobe flashes and weird partial slices for some reason the world has decided that this is totally fine almost all modern video footage amateur or professional has rolling shutter and even some theatrical movies don't seem to mind having these artifacts in the picture but that's not good enough for this channel we're gonna need to dig up a modern camera with global shutter okay let's do this now our blades are nice and straight and the flashes fill the frame as we ramp up the strobe speed the fan appears to slow down then absolute stillness it's cool but now let's try a non seizure-inducing version instead of speeding up a strobe light will slow down our cameras frame rate to dial in the effect same as in the original video at the default 24 frames per second things look pretty spinny by 18 frames per second they slow down and at exactly 15 frames per second the magic happens let's mark one of the blades in order to keep track of it we can see that it doesn't travel very far it pops just one place over on every frame that's 72 degrees every fifteenth of a second or three revolutions per second or a hundred and eighty rpm that math checks out is it not Australian recreational mathematics author comedian and YouTube personality Matt Parker that's right three times 16 is 180 nice anyway wait there's loads more interesting mathematics I've just made a whole video that when the blades of a helicopter align with the frame thanks Matt now the helicopter footage was shot at 25 frames per second but helicopter rotors also spin much faster between 250 and 600 rpm so here the blades probably swap over two or even three places every frame in our simulation we have to compensate for the slower fan by reducing the frame rate but the resulting illusion known as the wagon wheel effect is the same hope that clears it up if you want to see this replicated with an actual helicopter I guess you can check out Matt Parker's fancy video but if you want to try your own experiment with a ceiling fan and you need to hang one up don't waste money on a professional installation I rigged this bad boy all by myself and has been working just
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